1. Field of the Invention
The present invention relates to an unsaturated alkyd having a number-average molecular weight of at least 5000, an unsaturated polyester resin obtained therefrom and a method for the preparation thereof. Particularly, the present invention relates to an unsaturated alkyd having a number average molecular weight of at least 5000 with 10 mole % or more of its terminal group consisting of a carboxyl group, an unsaturated polyester resin obtained therefrom and a method for the preparation thereof.
2. Description of the Related Art
It is generally known to dissolve an unsaturated alkyd obtained by esterifying, with polyhydroxy alcohols, an .alpha.,.beta.-unsaturated polybasic acid or an anhydride thereof such as maleic anhydride or fumaric acid, which is used in combination with a saturated polybasic acid, an unsaturated alicyclic polybasic acid or an anhydride thereof in a monomer compolymerizable with the alkyd to form an unsaturated polyester resin.
Such unsaturated polyester resins are used for fiber-reinforced plastics (FRP), paints, casting and the like in a very wide range, and the annual output of the polyester resins has exceeded 200,000 tons.
Although unsaturated polyester resins are very important in the industrial field, the number-average molecular weight of the unsaturated alkyd which is a main component of the unsaturated polyester resins is at most about 1000 to 2500 as is common in conventional thermosetting resins and it is not an overstatement to say that there is no alkyd having a molecular weight of at least 3000.
The reason for this is that gelation makes it impossible to obtain a number-average molecular weight of the above value or more by the conventional direct esterification method using a polybasic acid or anhydride thereof and a polyhydroxy alcohol.
In some cases, attempts may be made to employ a conventional deglycol reaction, i.e., reaction at a high temperature of 250.degree. C. or more and a highly-reduced pressure of 1 mmHg or less in the presence of a metal compound catalyst. Although the inventors attempted to synthesize a high-molecular weight unsaturated alkyd under the above conditions, the attempt was a failure due to the occurrence of inevitable gelation even when a large amount of polymerization inhibitors were added.
A thermosetting unsaturated alkyd cannot be synthesized under general conditions for synthesis of thermoplastic polyesters. It is thus obvious that there is a fundamental difference between both types of polyesters, and there are no cases where synthesis of a high-molecular unsaturated alkyd is achieved by the above method.
For example, in the case of a thermoplastic polyester, the number-average molecular weight and weight-average molecular weight decrease after the optimum time under reaction conditions has passed, as shown in FIGS. 1 and 2. Namely, this indicates that a decomposition reaction takes place. In the case of an unsaturated alkyd, the number-average molecular weight becomes constant at a value slightly lower than the highest value, as shown in FIG. 3. However, the weight-average molecular weight of the unsaturated alkyd steadily increases as shown in FIG. 4. This shows that crosslinking of the unsaturated alkyd takes place at the same time as decomposition thereof. It is clear from the above fact that the thermoplastic polyester and unsaturated alkyd are essentially different in reactions from each other.
It has been known that, in order to increase the molecular weight of these heat curable alkyds, deglycolization was carried out under reduced pressure using an organic titanium catalyst such as tetraisopropyl titanate (i.e., Japanese Patent Laid Open Nos. 3-153719, 3-220232).
In contrast to the fact that low molecular weight unsaturated alkyds synthesized by conventional esterification have balanced amounts of carboxyl groups and hydroxyl groups, in methods where molecular weight enhancing is attempted by deglycolization almost all terminal groups of the resultant higher molecular weight alkyd necessarily change to hydroxyl groups.
Of course, the unsaturated alkyds mentioned above may be used in applications where the terminal hydroxyl groups may not cause any trouble such as in casting molds. However, there are applications where at least a part of the terminal groups must be carboxyl. That is the case, for example, where molding material is produced by increasing molecular weight through the reaction of terminal carboxyl groups and divalent metal oxides or hydroxides, such as SMC (Sheet Molding Compound), BMC (Bulk Molding Compound). However, attempts to esterify the alkyd by incorporating a conventional polybasic acid are not practical as there is a fear of lowering molecular weight as a result of decomposition of the unsaturated alkyd.
The inventors have therefore achieved a method for synthesizing the unsaturated alkyds of the present invention which can satisfy the above mentioned requirements by finding that carboxylation of the terminal hydroxyl groups can be achieved without causing any degradation of molecular weight, by adding at a desired temperature a polybasic acid anhydride with or without a free carboxyl group showing no volatile or sublimating properties, and then if required, followed by further treatment under reduced pressure below 5 mmHg.